This invention relates to a solid tantalum capacitor with a clean riser, and more particularly to such a capacitor wherein the area around the weld between the riser and an external lead is free of unwanted conductive material, and also more particularly to a method of making such a capacitor.
Prior art solid tantalum capacitors conventionally employ an anode of tantalum, a riser of tantalum joined to the anode and extending therefrom for attachment to an external lead, and dielectric and conductive cathode layers overlying the anode. The connection between the riser and the external lead of such a capacitor must be free of the several layers that overlie the anode. However, the common commercial methods for producing solid tantalum capacitors involve batch handling and pyrolytic conversions that result in deposition of at least some part of the several cathode layers onto the riser.
Prior art welding of an external lead to the riser through the overlying layers has been difficult to accomplish, and has often resulted in leakage paths between the riser (anode) and the solid electrolyte layer (cathode). The problem has become increasingly severe with the demands for lower inductance capacitors, which demands are met by shortening the riser, as by welding the external lead to the riser closer to the anode. However, in welding closer to the anode, there is greater chance for leakage by bridging from the riser to the cathode layers.
Various methods have been employed in the prior art to avoid leakage paths between anode risers and cathode layers, generally involving some form of mechanical removal of the overlying layers, e.g. sand-blasting of the risers. These prior art methods have been costly and time consuming, have not provided a clean weld area in a significant percentage of units, and frequently even have intermingled the overlying layers to the extent of forming rather than eliminating leakage paths.